<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script   src="https://code.jquery.com/jquery-1.12.4.js"   integrity="sha256-Qw82+bXyGq6MydymqBxNPYTaUXXq7c8v3CwiYwLLNXU="   crossorigin="anonymous"></script>
</head>
<body>

<div id="app">
    <input type="text" id="input"   />
    <div id="box" >

    </div>
</div>

<script>

    //当改变 input value的时候
    $('#input').keyup(function(){

        //获取 input value
        let inputValue = $('#input').val();
        //为box 赋值
        $('#box').text(inputValue);
    });

</script>

</body>
</html>